The present invention relates generally to a method and apparatus for transferring heat, and more specifically, for cooling a fuel cell stack.
Cooling systems are implemented in a variety of applications for cooling a heat source. Generally, cooling systems include a cooling fluid flowing therethrough, which undergoes phase changes to perform the cooling function. In particular, the cooling fluid cools the heat source via a heat transfer therefrom, whereby the cooling fluid is caused to vaporize from an original liquid form. The coolant fluid, in vapor form flows through a heat exchanger which is in heat exchange communication with a lower temperature source, such as ambient air. As the vapor flows through the heat exchanger heat exchange occurs from the vapor, thereby partially transforming the coolant fluid to its liquid phase. A condenser is also included for condensing the remaining vapor phase to the liquid phase. A large circulation pump is required for circulating the liquid coolant through the heat source and the components of the cooling system.
One such application that requires a cooling system is a fuel cell system. Fuel cells have been used as a power source in many applications, such as electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer electrolyte membrane having the anode catalyst on one face and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings formed therein for distributing the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode catalysts.
The term xe2x80x9cfuel cellxe2x80x9d is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are typically bundled together to form a fuel cell stack and are commonly arranged electrically in series. Each cell within the stack includes the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. By way of example, some typical arrangements for multiple cells in a stack are shown and described in U.S. Pat. No. 5,763,113.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2) or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. As such these MEAs are relatively expensive to manufacture and require certain conditions, including proper water management and humidification and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
The electrically conductive elements sandwiching the MEAs may contain an array of grooves in the faces thereof for distributing the fuel cell""s gaseous reactants (i.e., hydrogen and oxygen in the form of air) over the surfaces of the respective cathode and anode. In the fuel cell stack, a plurality of cells are stacked together in electrical series while being separated by a gas impermeable, electrically conductive, bipolar plate. The bipolar plate serves several functions including: (a) acting as an electrically conductive gas separator element between two adjacent cells; (2) distributing reactant gases across substantially the entire surface of the membrane; (3) conducting electrical current between the anode of one cell and the cathode of the next adjacent cell in the stack; (4) keeping the reactant gases separated in order to prevent auto ignition; (5) providing a support structure for the proton exchange membrane; and (6) in most cases, providing internal cooling passages defined by internal heat exchange faces and through which a coolant flow to remove waste heat from the stack. Various examples of a bipolar plate for use in PEM fuel cells are shown and described in commonly-owned U.S. Pat. No. 5,776,624.
Current fuel cell cooling systems are undesirably large, including the large circulation pump for circulating the liquid coolant through the fuel cell stack (i.e. heat source) to the heat exchanger where the waste thermal energy (i.e., heat) is transferred to the environment. The thermal properties of typical liquid coolants require a large volume to be circulated through the system to reject sufficient waste heat to maintain the stack operating temperature, particularly under maximum power conditions. For example, a PEM fuel cell stack operating at 80 KW and 50% efficiency with an operating temperature of 80xc2x0 C. will generate 80 KW of waste heat that must be rejected. However, since a maximum ambient air temperature of about 40xc2x0 C. can be utilized for heat rejection, a mass flow rate of approximately 2000-3000 grams/sec. of coolant must flow through the stack in combination with use of large heat exchanger areas to accommodate the required heat rejection. As is well known, the expense associated with large heat exchangers and the other cooling system components (recirculation pump, proportional mixing valves, PID controllers, etc.), combined with packaging constraints caused by physical size requirements of the components, have had a detrimental impact on widespread commercialization of fuel cell systems. Thus, a need exists to develop alternative fuel cell cooling systems which overcome the shortcomings of conventional cooling systems and assist in advancing the art.
The present invention relates to a method and apparatus for extracting waste heat from a heat source, such as a fuel cell stack, and rejecting the waste heat to the environment through a heat exchanger.
In one aspect, the method of the present invention extracts waste heat from a heat source, such as a fuel cell stack, by circulating a heat transfer fluid in a flow path between the fuel cell stack and the heat exchanger, and transferring heat from the fuel cell stack to the fluid by causing a portion of the fluid to vaporize. The energy required to vaporize the liquid coolant is significantly greater than the heat carrying capacity of the liquid.
In another aspect of the method of the present invention, the waste heat is rejected through the heat exchanger by separating the coolant discharged from the fuel cell stack into a vapor stream and a liquid stream, accelerating the vapor stream to supersonic speed, contacting the high velocity vapor stream with a portion of the liquid stream, and transferring the vapor stream momentum to the liquid. The resulting high temperature, high pressure liquid stream is delivered to the heat exchanger for heat rejection to the environment.
The apparatus of the present invention comprises a vapor separator for separating the coolant exiting the fuel cell stack into a liquid stream and a vapor stream, and a supersonic nozzle/ejector unit having a vapor inlet receiving the vapor stream and a liquid inlet receiving the liquid stream. The nozzle/ejector unit is operable to accelerate the vapor stream to supersonic velocity. The supersonic vapor stream accelerates the liquid stream as both travel toward the outlet of the nozzle/ejector unit. The high velocity vapor/liquid mixture enters the outlet of the nozzle/ejector unit which condenses the vapor and causes an increase in the coolant pressure and temperature. The high temperature, high pressure coolant is then delivered to the first heat exchanger where the fluid temperature is reduced to the operating temperature of the fuel cell stack. The fluid is then circulated to the inlet of the fuel cell stack. As such, the nozzle/ejector unit generates sufficient fluid pressure to drive circulation of the coolant through the entire system and further increases the heat rejection temperature of the fluid delivered to the heat exchanger for permitting reductions in the heat exchanger surface area and the mass flow rate of the coolant.
In accordance with one aspect of the present invention, the liquid stream from the vapor separator is returned to the inlet of the fuel cell stack and the liquid stream delivered to the liquid inlet of the nozzle/ejector unit is routed from a portion of the coolant flow downstream of the heat exchanger. As a related aspect, a second heat exchanger is disposed between the first heat exchanger and the liquid inlet of the nozzle/ejector unit for cooling the liquid stream prior to delivery to the nozzle/ejector unit. This arrangement is operable to establish a desired temperature difference between the vapor stream and the liquid stream delivered to the inlet side of the nozzle/ejector unit.
The present invention utilizes vapor cooling of a heat source, such as the fuel cell stack, to provide improved control over the stack operating temperature. In addition, the enthalpy of the vapor is used to raise the heat rejection temperature of the coolant delivered to the primary heat exchanger and its fluid pressure is used to circulate the coolant. This permits elimination of the large circulation pump used in conventional cooling systems which significantly improves the overall efficiency of the fuel cell system. The invention is also adaptable for use in a variety of systems where heat transfer occurs, and it is desired to decrease pumping power and increase the temperature at which heat is rejected.